Resorcinol, resorcinolic derivatives and resorcinol-formaldehyde resins have been used in the rubber industry as rubber compounds and adhesives. These resorcinolic compounds are unique materials in rubber compounding, since they act as thermosetting and vulcanizing plasticizers. They are very efficient plasticizers for rubber during the processing operations. The use of these compounds allows easier processing, higher loading and excellent extrusions of the rubber compounds.
Resorcinol and resorcinol based derivatives and resins, which act as methylene acceptors, have thermosetting properties that, upon curing, form a resin network within the rubbery polymer by reacting with various methylene donors. This results in increased hardness, abrasion resistance, aging resistance, solvent and oil resistance, and stiffness, and also gives a much improved finish to the cured rubber stock. This combination of plasticizing and reinforcing action is rare for a single material used in rubber compound formulations. In addition, resorcinolic compounds may act as antioxidants when used in natural rubber.
The network formation during rubber curing is more effective with phenolic methylene acceptor compounds having meta-substitution capability than with other compounds due to their high reactivity towards methylene donors. Examples of such phenolic compounds include resorcinol, phloroglucinol and m-aminophenol. The use of phloroglucinol and m-aminophenol in rubber compounding formulations is limited due to their high cost and melting points. Therefore, the most commonly employed methylene acceptor is resorcinol, due to its comparatively low cost and high reactivity. Resorcinol has three reactive sites, namely at C2, C4 and C6, indicated in formula 1 by the *: 
The high reactivity of resorcinol towards conventional methylene donors such as formaldehyde, hexamethylenetetraamine (HMTA) and hexamethoxymethylmelamine (HMMM), etc., is due to the presence of three reactive sites, meta with respect to each other, on the resorcinol molecule. The presence of two hydroxyl groups, meta with respect to each other, on the molecule further enhances the reactivity of the molecule towards methylene donors.
Though resorcinol provides enhanced physical, mechanical and adhesion properties in the cured rubber compounds, fuming associated with this material can be a problem to the tire industry. To overcome this problem, the tire industry needs compounds such as modified resorcinolic derivatives and resins that do not produce volatiles, such as those produced by resorcinol at mixing temperatures above 110° C. In addition to low volatility, the new resorcinolic compounds should have a reactivity similar to resorcinol in the rubber compound.
When the resorcinol is derivatized by attaching functional groups to either the benzene ring or hydroxyl groups, the reactivity of the resulting resorcinolic derivatives towards the methylene donor compounds is greatly reduced or altered. For example, substitutions at the 2, 4 or 6 positions of the benzene ring reduce the number of reactive sites for methylene donor interaction. Also, monoether or monoester derivatives have reduced reactivity towards methylene donor compounds as compared to resorcinol.
Resorcinol monobenzoate and derivatives of resorcinol such as resorcinol monorosinate, resorcinol diphenyl ether, resorcinol monomethyl ether, resorcinol monoacetate, phloroglucinol and derivatives used in the rubber composition are disclosed in U.S. Pat. No. 4,605,696. For example, to overcome the fuming of resorcinol, monoester derivatives of resorcinol, such as resorcinol monobenzoate, were used in the rubber compound. While improved dynamic mechanical properties were observed for the cured rubber, the monoester derivatives were less reactive than resorcinol due to the presence of an ester group substituent.
U.S. Pat. No. 4,892,908 discloses a keto derivative of resorcinol, namely benzoylresorcinol, which can be used as a low-fuming reinforcing material in the rubber compound. But, benzoylresorcinol has two reactive sites and a high melting point as compared to resorcinol.
U.S. Pat. No. 5,049,618 discloses a vulcanizable rubber composition, which comprises rubber, a filler and N-(3-hydroxyphenyl)maleimide. N-(3-hydroxyphenyl)maleimide has three active sites for methylene donor compounds but is believed to have a higher melting point than resorcinol.
Thus, while modifications to the resorcinol molecule are expected to lower the fuming characteristics of resorcinol, they also affect the networking efficiency during curing. Therefore, it is important to develop resorcinolic compounds/derivatives that can at least maintain the reactivity of resorcinol, if not impart higher reactivities towards the methylene donor compounds. Increased reactivity of resorcinolic derivatives is expected to speed up the network formation during rubber vulcanization, which ultimately enhances the physical and mechanical properties of the cured rubber.